1. Field of the Invention
This invention relates to radar systems and particularly to an improved moving target indicator and clutter tracking system in which clutter offset at different doppler frequencies is cancelled in different moving target indicator stages.
2. Description of the Prior Art
In a moving target indicator (MTI) system, a coherent oscillator is provided and is locked in frequency to the ground clutter or is tracking the ground clutter in the first canceller stage so that other types of clutter such as weather or chaff at a different doppler frequency offset are not sufficiently cancelled either in that or in subsequent stages. Complex cancellers using feed forward and feedback techniques have been developed for clutter cancellation to give shaped filter characteristics such as butterworth or chebyshev responses. When these arrangements are utilized in clutter environments in which two different doppler frequencies are encountered, an attempt is made to widen the MTI notch so that both clutter responses will be contained in the notch. However, these complex shaped filters have only a limited range of adjustment so that in many situations a required doppler offset between two types of clutter will exceed the adjustment range resulting in the clutter being improperly cancelled. Another type of clutter canceller utilizes a clutter tracker which for example may vary the frequency of the coherent oscillator in response to the cluter doppler frequency. It has been found that when the different velocity clutters overlap in range, the clutter tracker will track the power mean of the two clutter returns thus giving a relatively poor performance on the smaller clutter return. It would be a substantial advantage to the radar art if a clutter tracking system was provided that allows second or subsequent stages of the MTI canceller to independently track clutter energy at different doppler offset frequencies.